1. Field of the Invention
The present invention relates to a process for oligomerizing olefins of from 2 to 8 carbon atoms or mixtures over a nickel-containing catalyst at temperatures of from 0.degree. to 200.degree. C. and pressures of from 1 to 70 bar absolute.
2. Description of the Background
Olefins of from 2 to 8 carbon atoms or mixtures thereof, in particular olefins of from 2 to 5 carbon atoms, are available in large quantities from refinery crackers, and therefore are important raw materials for the entire petrochemical industry. For instance, polymers of ethylene, propylene and 1-butylene or mixtures thereof have found numerous applications.
Equally, however, the branched oligomers of olefins of from 2 to 8 carbon atoms, prepared by acid catalysis, have become important in industry. For instance, polymer gasoline has been produced from C.sub.3 /C.sub.4 -olefin mixtures for decades, and various fractions isolated from polymer gasoline are used as starting materials, for example for PVC plasticizers and detergents.
The importance of petrochemical products produced from polymer gasoline fractions is exceeded by that of products produced from more linear olefin oligomers because, for example, such detergents or detergent bases are more biodegradable or because such PVC plasticizers have inter alia lower viscosities and improved low temperature properties, yet a similar vapor pressure. Less branched oligomers of from 10 to 16 carbon atoms produced from C.sub.5 /C.sub.6 -olefin cuts are highly suitable for use as diesel motor fuel component. This aspect is of great importance in those countries where the motor fuel supply is chiefly based on coal.
The more linear oligomers of lower olefins are obtainable by reacting the latter at temperatures of about 0.degree.-200.degree. C. and pressures of about 1-70 bar not only over homogeneous, but also over heterogeneous catalysts, when the active component is predominantly nickel. However, there are other possible catalytically active metals, for example ruthenium (G. Braca, G. Slzana; La Chimica e l'Industria, 56 (1974), 110-116), palladium, as disclosed in U.S. Pat. No. 4,436,946, and copper, cobalt, iron, chromium and titanium as disclosed in GB Patent 824,002. However, only the nickel-containing catalysts have become industrially important.
DE Patent 2,855,423 discloses a homogeneous catalyst system consisting of the nickel(II) salt of octanoic acid, ethylaluminum dichloride and a free fatty acid. A catalyst system of this kind is also used in the only homogeneously catalyzed olefin oligomerization process of industrial importance (DIMERSOL.RTM.) (Y. Chauvin et al., Chemistry and Industry, 1974, 375-378). Homogeneously catalyzed processes for oligomerizing olefins are very cost-intensive because of the technically complicated process of removing the catalyst system and, what is more, it is necessary to provide for a complicated disposal system for the inevitable waste product formed in the course of the destruction of the catalyst.
Besides homogeneous catalysts the prior art also discloses numerous heterogeneous catalysts based on nickel and silicon which frequently contain aluminum in addition and are prepared in various ways. DD Patent 160,037, for example, discloses the preparation of an Ni/Al precipitation catalyst on SiO.sub.2 as carrier material. Other catalysts are obtained by exchanging the positively charged particles, such as protons or sodium ions, present on the carrier surface for nickel ions. A wide variety of different carrier materials are used in the catalysts, for example, amorphous aluminum silicate according to R. Espinoza et al Appl. Cat. 31 (1987), 259-266, crystalline aluminum silicate according to DE Patent 2,029,624, zeolites of the ZSM type according to NL Patent 8,500,459, an X zeolite according to DE Patent 2,347,235, X- and Y-zeolites according to A. Barth et al., Z. Anorg. Allg. Chem. 521 (1985), 207-214, and a mordenite according to EP-A-0,233,302.
It is known that nickel-containing catalysts are sensitive to a wide range of catalyst poisons. Such catalyst poisons are inter alia polyunsaturated hydrocarbons, for example, propyne or butadiene, halogen compounds, oxygen compounds, e.g. water or alcohols, sulfur compounds, e.g. hydrogen sulfide, carbon oxysulfide, thioalcohols and thioethers, and also nitrogen compounds, for example, amines present in the FCC C.sub.4 -hydrocarbon cut (FCC is the abbreviation for fluid catalytic cracker), or traces of butadiene-extracting agents, e.g. acetonitrile or N-methylpyrrolidone in SC raffinate I (SC being the abbreviation for steam cracker). The mechanism of these catalyst poisons, however, is not well known, but it is suspected that their effect results from the fact that they are more strongly adsorbed on the catalytically active centers than the olefins to be oligomerized. The presence of such catalyst poisons in the olefins to be oligomerized gradually reduces the activity of the catalyst.
Consequently, processes are known in which various compounds are removed from a hydrocarbon mixture. According to DE Patent 2,057,269, prior to the oligomerization, polyenes are preferably catalytically hydrogenated to the corresponding monoolefins to an extent of up to 75%. U.S. Pat. No. 4,153,638 teaches that, after catalytic hydrogenation, the diolefin content should be below 1% by weight. Since the nickel-containing oligomerization catalysts, in general, also show hydrogenation activity, it is possible according to EP Patent 0,091,232, to convert the diolefins into the corresponding olefins by passing the hydrogen-saturated hydrocarbon feedstock mixture over the oligomerization catalyst.
According to U.S. Pat. No. 4,153,638, any dissolved water present in the hydrocarbon feedstock mixture can be removed down to levels of less than 10 ppm by weight by means of customary drying agents, for example, a molecular sieve having a pore diameter of 3 angstroms or activated bauxite. Other high-boiling oxygen compounds are removed according to DE Patent 2,057,269 as the bottom product in a distillation of the hydrocarbon mixture.
Sulfur compounds can be removed by an alkali wash. and nitrogen compounds by a water wash, the preference of U.S. Pat. No. 4,153,638 being hydrocarbon oligomerization mixtures containing less than 1 ppm by weight of sulfur and 0.3 ppm by weight of nitrogen. However, the prior art processes do not achieve a thorough removal of such compounds. For instance, a refinery-produced propene/propane mixture (about 75% by weight of propene and about 25% by weight of propane) still contains, even after desulfurization of high-boiling sulfur compounds, for example, dimethyl sulfide (boiling point: 38.degree. C.) or dimethyl disulfide (boiling point: 109.degree. C.), in concentrations of less than 0.5 ppm by weight. However, these levels of sulfur compounds are sufficient to shorten the lifetime of the oligomerization catalyst to the point of the process becoming uneconomical.
A further example are C.sub.4 -hydrocarbon cuts which, despite prior distillation may still contain trace amounts of high-boiling oxygen compounds, for example methyl tert-butyl ether, tert-butyl alcohol and acetone. The same is true of C.sub.5 -hydrocarbon cuts, for example pyrolysis gasoline. Since, however, catalyst poisons are effective even in trace amounts, the abovementioned prior art rough purification processes are not sufficient to ensure the removal of catalyst from the hydrocarbon feedstock mixture for the oligomerization of olefins. For this reason and the consequent too short catalyst lifetime, the catalytic processes for oligomerizing olefins, in particular the heterogeneously catalyzed processes, have failed to become established in practice.